Good Question. There is a reason why Krishna let the Draupadi Vastraharan sequence happen. He knew what happened everywhere because He is the all-knowing, all-pervading Lord Vishnu Himself. This answer is based on the Mahabharata Tatparya Nirnaya of SriMadhvacharya. The few reasons why Krishna allowed the Draupadi Vastraharan to happen are:
TO EXPOSE THE EVIL NATURE OF DURYODHANA AND THE KAURAVAS
From the beginning itself, Duryodhana and his brothers had been troubling the Pandavas. Their major aim was to kill Bheemasena because He was the strongest among all the Pandavas and was their backbone. So, Duryodhana thought to kill Bheemasena first, and then he would look for the other Pandava brothers. For that, Duryodhana tried a lot to kill Bheemasena by feeding him a laddoo that had halahala visha in it. Shakuni had pleased Shukracharya and had gotten some poison from him. That didn’t work on Bheemasena. Then he tried to put snakes on Bheemasena to kill him. But Bheemasena broke the fangs of the snakes and killed the owner of the snake.
He even threw Bheemasena into the Pramaanakoti, a deep place within the Ganga River. From there, Bheemasena also came out unaffected. These were some incidents that weren’t very famous at that time. He even sent and tried to kill the Pandavas in the Vaaranavata Laakshaagraha, but nothing happened. Then, people also cursed Duryodhana and all. But now, when they tried to do Draupadi Vastraharana, it was not just the vastraharana of the princess of Panchala, or the daughter of Drupada, but the queen of the whole Aryavarta or Bhaaratadesha.
Krishna let that happen because He wanted to expose the evil and deceitful nature of Duryodhana, Dushasana, Karna, and Shakuni. Why? So that people would know how evil they were. This was necessary because, in the future, the way in which the Kauravas, Bhishma, Drona, and Karna died should not be disputed and should be clear to the people as to why these people were killed unjustly. Actually, what Krishna did in the Mahabharata yuddha is completely correct because Adharmik people must be killed by using Adharma, and that is Dharma. If you kill Adharmis by Dharma, it is a sin. The Paingi Shruti itself says this.
TO DISTRIBUTE THE PUNYA OF THE RAAJASUYA YAGYA TO THE PAANDAVAS
Dharmaraja did the Raajasuya yagya. But the fruit of the Raajasuya yagya was such that Dharmaraja was not capable of sustaining that much punya. Therefore, to distribute it among the Pandavas, Krishna made this happen. SriMadhvacharya says that the fruit of the Raajasuya Yagya is Brahma Padavi. Also, Bheemasena is Vayu avatara. The Garuda Purana itself says:
अतो रोचननामासौ मरुदंशः प्रकीर्तितः ।
रामावतारे हनुमान्रामकार्यार्थसाधकः ॥
स एव भीमसेनस्तु जातो भूम्यां महाबलः ॥
He (the cosmic soul) is known as the amsha (portion/incarnation) of Marut (Vayu). In the Rama avatara, he became Hanuman to accomplish the divine tasks of Shri Rama. The very same entity was later born on Earth as the highly powerful Bheemasena.
[Garuda Purana]
So, here it was Bheemasena Himself who was Hanuman in His previous birth (as stated in the Garuda Purana). Yes, Hanuman is the elder brother to Bheemasena in one sense, because both are known as the sons of Vayu. But both are also incarnations of Vayudeva. Plus, there is no difference between Vayudeva and His avatars. So, Hanuma is the same as Vayudeva; Bheemasena is the same as Vayudeva. So, this Vayudeva is eligible for Brahma Padavi. For this, the Valmiki Ramayana is itself the proof. It says:
सर्वासु विद्यासु तपोविधाने प्रस्पर्धते ऽयो हि गुरुं सुराणाम् ।
सो ऽयं नवव्याकरणार्थवेत्ता ब्रह्मा भविष्यत्यपि ते प्रसादात् ॥
Indeed, in all branches of learning and in the performance of austerities, this Hanuman rivals even Brihaspati, the preceptor of the Gods. This very person, who is the knower of the deep meaning of the nine systems of grammar, will definitely become Brahma in the future by your grace.
[Valmiki Ramayana, Uttara Kanda, Sarga 36, Verse 48]
Thus, it is Vayudeva who is eligible for the Brahma Padavi. The previous Brahma will attain Moksha. And Bheemasena is Vayudeva’s incarnation. Even the Vayu Purana also states that Bheemasena is Vayudeva’s avatara. So, the Raajasuya yagya’s maximum punya should reach Bheemasena and Draupadi, because Draupadi is the incarnation of Bhaarati Devi, the wife of Vayudeva. So, She will be the future Sarasvati Devi. So, therefore, for the transfer of the maximum punya to Bheemasena and Draupadi, Krishna made that happen. That’s why Dharmaraja was silent during the Draupadi Vastraharana episode, and because he was silent, the punya got transferred to the other Pandavas.
Therefore, it was Bheemasena who thoroughly criticized the Kauravas. He even criticized Dharmaraja by asking Sahadeva to bring fire to burn Dharmaraja’s hands. Arjuna stops Bheemasena and calms Him down. Bheemasena says to Arjuna, “I won’t do this literally, but I am doing this because, even though elders commit anything bad, we must not keep silent but should criticize them. I am just criticizing him, but won’t do that literally.” At this, Duryodhana says, “If you accept that everything which is going on here happened due to Dharmaraja, then I will leave all of you.” At this, Bheemasena said, “Nobody is as Dharmatma as my elder brother Dharmaraja. Plus, I am the elder brother to all of you. If you have guts, then come, fight me.”
He even vowed to break the thighs of Duryodhana and tear the chest of Dushasana very aptly (not as a fanatic, but I felt it was very apt). He did criticize the Kauravas. Plus, Draupadi was thoroughly devoted to Krishna and remembered Krishna throughout the Vastraharana episode. Therefore, the maximum punya was given to Bheemasena and Draupadi. Plus, the Pandavas had remained silent out of respect for Dharmaraja. The Pandavas, led by Bheemasena and Arjuna, could have destroyed the Kauravas, but Dharmaraja, with his eye-coordination, had stopped them and asked them to sit down. Because previously he had vowed that he wouldn’t scold anyone, wouldn’t abuse anyone, wouldn’t refuse anyone, and wouldn’t criticize anyone.
TO TRANSFER THE TAPAS SHAKTI OF THE KURU VRUDDHAS TO THE PANDAVAS:
The Kuruvruddhas like Bhishma, Drona, Krupa, Dhritarashtra, etc. had remained silent during the Draupadi Vastraharana. Why was it like that? Because SriMadhvacharya explains that Kali Purusha had entered all of them. Only a few people were there who were not affected by Kali Purusha. These were the Pandavas (except Dharmaraja), Draupadi, Kunti, and Vidura. Rest all were full of Kali Purusha’s Avesha. SriMadhvacharya mentions this in the Mahabharata Tatparya Nirnaya. But why could Kali Purusha enter people except some people? Because all these, like Kunti, the rest of the Pandavas, Draupadi, and Vidura, had Nishthura Dharma. Nishthura Dharma means, Adharmi must be punished, and Dharma must be followed at any time. But Dharmaraja had Daakshinya Dharma, which means his heart used to melt at anyone’s plea or cry.
So, because Kunti, Vidura, etc. had perfect Kathina/ Nishthura Dharma nishtha, they were not affected by Kali Purusha baadha. So, due to this, others except these people were affected by Kali Purusha Avesha and hence remained silent. Duryodhana himself was Kali Purusha. The Mahabharata itself mentions it. So, he was now controlling the whole dyuta sabha except these few people. So, since these elders or kuruvruddhas were silent, for that reason, the tapas shakti had transferred to the Pandavas. These Kuruvruddhas were great tapasvis and knowledgeable personalities. But due to this one mistake, their punya or tapas shakti was transferred to the Pandavas.
Thus, the Pandavas had their own tapas shakti and punya, plus they had the Kuruvruddhas ’ punya with them, plus they too had the Rajasuya’s yagya’s punya (maximum to Bheemasena and Draupadi). So, that’s another reason why Krishna let the Draupadi Vastraharan episode happen. Plus, Krishna Himself protected Draupadi by giving Her Akshayavastra.
TO INCREASE THE SINS OF DURYODHANA AND KAURAVAS:
There is a rule that, until a fixed number of papas are done, they aren’t killed. So, Duryodhana was already a sinner. At birth, the Mahabharata mentions that he cried like a donkey. The other donkeys from the surroundings had gathered and were braying aloud on hearing Duryodhana’s cry. And at that time, Vidura himself tells Dhritarashtra to kill that baby because it can lead to the destruction of the whole Kuruvamsha. But Dhritarashtra wanted to be reputed as the father of 100 sons. So, he didn’t kill him at that time. From the beginning, Duryodhana was jealous and was a sinner.
Because of this act of Duryodhana, he should not just go to hell and come back, but he should receive eternal hell itself, where he stays for an unlimited period of time, experiencing eternal sorrow. So, that many sins he must incur. So, since they incurred a lot of sin by their acts, they were sent to hell. Though Duryodhana first attained heaven because he had some sukha prarabdha, he was pushed to Andha Tamas hell afterwards. Plus, by this, Krishna wanted to warn us that, ‘Now that you have seen the cruel acts of these sinners, don’t accuse me of what I did in the Mahabharata Yuddha. Because Adharmis must be killed by using Adharma and not Dharma. So, what I (Krishna) did in the Mahabharata yuddha was completely correct.’
NOTE: Thus, we must not just read the Mahabharata as a political history of the fight between the Kauravas and the Pandavas. But we should see the spiritual meaning behind it, because it is not just a normal history book, but our divine history, which is real. The Puranas are for the support the incidents that are not mentioned in the Itihasas. They help in connecting the dots if we get any confusion in the Itihasas. Therefore, I also used the Puranas. SriMadhvacharya, had read the Mahabharata granthas across India, and had identified the corrections and interpolations, and then wrote the Mahabharata Tatparya Nirnaya. Therefore, understanding the spiritual meaning behind every incident is important.
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!